Graphite offers several advantages as a material of construction for equipment used in difficult environments. In particular, graphite has an excellent heat transfer coefficient and exhibits excellent thermal stability in non-oxidizing environments. However, graphite is porous, and, when used for the construction of heat exchangers, the pores must be filled to prevent cross-contamination of the liquid streams. Presently, porous graphite tubes and blocks used for heat exchanger applications are impregnated with thermoset resins of the phenolformaldehyde or furan-formaldehyde type. This is accomplished by forcing the oligomeric resin into the graphite pores under pressure, and heating to cure. Three to four cycles are required to obtain a product which does not show permeation in five minutes under 115 psi (0.79 MPa), a quality control test.
In addition to sealing the pores, the resin impregnants provide useful improvements in the strength of the graphite. However, the use of the organic resin impregnants limits the service temperature of impervious graphite heat exchangers to less than 177.degree. C. Although it is possible to impregnate graphite heat exchangers with a fluorocarbon resin to provide service temperatures of up to 232.degree. C., still higher operating temperatures are considered desirable.
Composite materials comprising graphite and silicon carbide have been known for many years. U.S. Pat. No. 1,483,507 is directed toward such articles and a method for their manufacture. The latter calls for the incorporation of silicon and graphite and an inert refractory with an adhesive. A molded article thereof is heated to volatilize and diffuse the silicon into the pores of the graphite. It is next immersed in caustic soda to form a silicate which, when subsequently heated, exudes to the surface and forms a glaze.
U.S. Pat. No. 2,013,625 is also directed toward refractory articles which can comprise graphite and certain metallic substances such as metal, metal alloys and metalloids. When the article is heated, the metallic substances present at the surface are oxidized to form molten and viscous glass which provides a protective coating. The metallic substance can be distributed throughout the material from which the article is manufactured or by concentrating it at the surface such as by painting or spraying an oil suspension of the material onto the article or mold in which the article is made.
U.S. Pat. No. 3,264,222 is directed toward a method of manufacturing refractory products of amorphous carbon or graphite. The latter material is mixed with a refractory metal-containing material such as zirconium hydride. The mixture is compressed under high pressure and rapidly heated to about 2000.degree. C. to melt the zirconium and cause it to flow into the pores of the graphite. Continued heating carburizes the zirconium.
U.S. Pat. No. 3,140,190 provides a method of making ceramic refractory compositions having a graphite base. Powdered graphite is blended with a ceramic additive and a liquid carbonaceous binder. The blend is then compressed and shaped and heat treated to resinify the binder and then graphitize it. The ceramic additives include molybdenum disilicide and titanium diboride which form glassy ceramic protective coatings.
Finally, U.S. Pat. No. 4,539,301 discloses thin, flaky graphite-containing refractories useful as lining materials for steel making furnaces. The material is obtained by refining and purifying a naturally occurring flaky graphite to obtain a specific thickness and flat surface layer size. These graphites are then conventionally mixed with known refractory materials to form refractory articles under pressure and heat.
As can be seen, generally each of the patents provides for the mixing of the graphite and ceramic material together to form an article by heating to high temperatures. While U.S. Pat. No. 2,013,625 allows for the ceramic or glassy material to be applied to the surface of the graphite article, the materials employed do not fill the pores of the graphite totally, but only at the surface. Thus, the art has not recognized heretofore a relatively simple process whereby a porous graphite structure can be impregnated with a glass precursor dispersion or solution which forms glasses at relatively moderate conditions.